This project combines the molecular and genetic aspects of the Drosophila system to study cell adhesion, growth control, and tumor suppression. The Drosophila fat gene, recently isolated and characterized as a member of the cadherin gene superfamily, is the focus of this study. Cadherins were first identified in vertebrates, where their ability to mediate cell aggregation and sorting in vitro, coupled with their dynamic pattern of expression in vivo, suggested a role for them as mediators of differential cell adhesion. Changes in cadherin expression frequently correlate closely with important developmental events, such as compaction of preimplantation embryos, implantation, migration to mesodermal cells, and differentiation of somites. The recent cloning of two different members of the cadherin gene superfamily from Drosophila DNA, and their identification as the fat and dachsous loci, now enables the role of cadherins in vivo to be examined directly. Viable mutations in these two loci cause defects that are consistent with the idea that cadherins are regulators of morphogenesis. Moreover, there is a well-defined genetic interaction between the two loci, in which mutations in the dachsous locus affect the phenotype of a dominant allele of the fat locus, called Gull. This genetic interaction provides a powerful tool to study the mechanism by which different members of the gene family interact at a molecular level. The most striking feature of these genes is the fact that recessive lethal fat mutations cause a loss of control over cell proliferation, resulting in large, tumor-like masses of tissue. This phenotype demonstrates that the wild-type fat protein functions as a tumor suppressor, consistent with the proposed role of cadherins in vertebrates. The goals of this proposal are to (1) test the hypothesis that the fat protein mediates adhesion, by transfecting non-adhesive Drosophila cells with fat DNA under control of an inducible promoter; (2) raise and utilize antibodies to the fat protein, to test the hypothesis that it is an extremely large cell surface protein concentrated at points of intercellular contact; (3) test the hypothesis that a truncated fat protein is the cause of the dominant Gull mutant phenotype, in order to better understand the fat-dachsous interaction; (4) transform flies with wild-type and altered constructs of the fat gene, in order to test the hypothesis that the fat gene is necessary and sufficient to rescue the mutant fat phenotype, and to refine our analysis of the dominant Gull mutation; (5) transform flies with fat deletion constructs and fat- dachsous chimeric constructs, to identify regions of the fat protein responsible for the tumor suppressing function. These last experiments take advantage of the fact that fat and dachsous encode closely related proteins, with the same pattern of expression, and yet only fat has a tumor suppressing function. We hope to use this difference to functionally dissect the tumor suppressing capability of the fat gene product. The recent demonstration in vitro that highly malignant mammalian tumor cell lines lose their invasive potential following the introduction and expression of the vertebrate E-cadherin gene suggests that our work in the fly system will have a broad application.